Highly insulated inductive data couplers

ABSTRACT

There is provided an inductive coupler for coupling a signal to a power line. The inductive coupler includes a magnetic core for placement about the power line, and a coil wound around a portion of the magnetic core. The coil includes a coaxial cable having a non-insulative outer layer coupled to a stress cone. The non-insulative outer layer is at an electrical potential about equal to that of the power line when the inductive coupler is installed on the power line.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 10/688,262, filed Oct. 17, 2003 now U.S. Pat. No. 7,109,835, whichclaims priority of U.S. Provisional Patent Application Ser. No.60/419,174, filed on Oct. 17, 2002, the content of which is hereinincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to power line communications, and moreparticularly, to a data coupler being insulated in a manner thatminimizes voltage breakdowns.

2. Description of the Related Art

An inductive coupler for power line communications couples a data signalbetween the power line and a communication device such as a modem. Theinductive coupler may suffer from insulation breakdown or partialdischarge at unsuitably low line voltages. Breakdown or partialdischarge will generally occur at a location within the coupler where anelectric field is concentrated in an insulating material or where anexcessively high field is created through the air.

FIG. 1 shows a cross-section of a prior art inductive coupler. A powerline 800, e.g., a phase line, serves as a primary winding for theinductive coupler, and thus passes through an aperture of a magneticcircuit with a core configured with an upper core portion that includesa core section 805 and a lower core portion that includes a core section810, and air gaps 830 and 835. A secondary winding 820 also passesthrough the aperture, surrounded by an insulating material 825. Powerline 800 touches core section 805 at a contact point 855, whilesecondary winding 820 is grounded. Core sections 805 and 810 are made ofa magnetic core material. An electric field inside of core sections 805and 810 depends on conductivity and permittivity of the core material.

For the case of power line 800 being bare, the full phase voltage isapplied to the coupler, specifically between contact point 855 andsecondary winding 820.

Referring to FIG. 2, for the case of power line 800 being covered withinsulation, there is shown power line 800 having insulation 860 thatcontacts core section 805 at a contact point 865. A capacitive voltagedivider is formed between (a) a capacitor formed between power line 800,insulation 860, and core section 805, and (b) a capacitance betweencontact point 865 and secondary winding 820. The voltage stress betweencontact point 865 and ground is then less than the full phase voltage.

A plane where secondary winding 820 exits core section 810, core section810 presents a sharp corner. In general, there may be two locationssusceptible to ionization and voltage breakdown, (1) an air path 840between power line 800 and insulating material 825, and (2) a regionbetween of the corners of core section 810 and the exiting of secondarywinding 820 from core section 810.

Air path 840 is susceptible to ionization and voltage breakdown, asfollows. Insulating material 825 is likely to be constructed from aplastic or other material with a permittivity of 2.5-3.5. A capacitivevoltage division of a voltage difference between power line 800 andsecondary winding 820 will place most of the voltage difference in airpath 840, and relatively little of the voltage difference across aninsulation path 850. The insulating capability of air is inferior tothat of plastic or other insulating material, so as voltage on powerline 800 increases, a breakdown is most likely across path 840.

FIG. 3 shows a horizontal cross section drawn through a secondarywinding 820 such as that shown in FIG. 1. The lower core portion isshown as being configured with a plurality of core sections, namely coresections 810, 811, 812 and 813. Secondary winding 820 passes throughcore sections 810, 811, 812 and 813. Regions 1000, 1005, 1010 and 1015represent regions of electric field concentration, and might causeinitial insulation breakdown at a voltage on power line 800 that is muchlower than desired.

SUMMARY OF THE INVENTION

An embodiment of an inductive coupler for coupling a signal to a powerline includes a magnetic core for placement about the power line, and acoil wound around a portion of the magnetic core. The coil includes acoaxial cable having a non-insulative outer layer coupled to a stresscone. The non-insulative outer layer is at an electrical potential aboutequal to that of the power line when the inductive coupler is installedon the power line.

Another embodiment of an inductive coupler includes (i) a magnetic corehaving an aperture through which a power line is routable, (ii) a cablehaving a center conductor routed through the aperture, and having aportion external to the aperture, (iii) a stress cone through which thecenter conductor is routed, and (iv) a semiconducting coating that (a)encapsulates the magnetic core and the portion of the cable that isexternal to the aperture, and (b) contacts the stress cone. Thesemiconducting coating contacts the power line when the inductivecoupler is installed on the power line.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-section of a prior art inductive coupler,perpendicular to a power line.

FIG. 2 shows a cross-section of another embodiment of a prior artinductive coupler.

FIG. 3 shows a horizontal cross section drawn through secondary windingof an inductive coupler such as that of FIG. 1.

FIG. 4 is a cross section of a highly insulated inductive data coupler,perpendicular to a power line.

FIG. 5 shows a horizontal cross section drawn through a lower coreportion of a highly insulated inductive data coupler, such as that ofFIG. 4.

FIG. 6 is a vertical cross-section through an arrangement employing aninductive coupler having a semiconductive coating.

FIG. 7 is a cross section of a high voltage inductive data coupler thatincorporates a cable as a secondary winding.

DESCRIPTION OF THE INVENTION

A highly insulated inductive data coupler, in accordance with thepresent invention, virtually eliminates high electric fields through airpaths, and limits those fields to locations filled with dielectricmaterial. Rounded geometries are employed on all energized bodies toeliminate any pointy features that might generate a high local field.Also, upper and lower core portions are placed inside a single commonequipotential envelope, making the coupler indifferent to dielectricproperties of magnetic cores, and eliminates electric fields within thecores and between upper and lower core portions.

FIG. 4 is a cross section of a highly insulated inductive data couplerin accordance with the present invention. The coupler includes amagnetic core for placement about power line 800. The magnetic core isconfigured with an upper core portion that includes core section 805 anda lower portion that includes core section 810. The designation of coreportions as being “upper” and “lower” merely refers to their respectivepositions in the drawings of the present disclosure, and suchdesignation is not necessarily descriptive of an actual physicalrelationship of the core portions. Secondary winding 820 is connected toa communication device (not shown) such as a modem, and thus, thecoupler enables a data signal to be coupled between power line 800 andthe communication device.

Each of core section 805 and core section 810 are encapsulated in bootsor coatings 900 and 905 made of a semiconductor material. Examples ofsuitable semiconductor materials are plastics or rubbers impregnatedwith graphite or silicon carbide to provide a desired bulk resistivity.An electrical contact 910 is made between coating 900 and coating 905.Core sections 805 and 810 and coatings 900 and 905 thus become a single,essentially equipotential body.

A surface 915 of insulating material 825 is covered with asemiconducting coating 945, which overlaps coating 905 and makeselectrical contact with coating 905. The potential of coating 945 isthus made essentially equal to the surface of power line 800,eliminating or greatly reducing the voltage across an air path 940. Thispermits an inductive coupler that includes secondary winding 820 andcore sections 805 and 810, and employs power line 800 as a primarywinding, to be safely used on higher primary voltages than would bepossible without semiconducting coating 945.

FIG. 5 shows a horizontal cross section drawn through a lower coreportion of a highly insulated inductive data coupler, such as that ofFIG. 4. The lower core portion is, in turn, configured with a pluralityof core sections, namely core sections 810, 811, 812 and 813. Theinductive data coupler of FIG. 5, when compared to that of FIG. 3,experiences a reduction of field concentration in region 1105, ascompared to region 1000, at the exit of the secondary winding 820 fromcore section 813. Coating 905 is equipped with a rounded profile 1100,which provides a rounded extension to the side of core section 813.Rounding the shapes of energized bodies, such as core section 813,reduces the maximum electric field in region 1105 for a given voltagecarried on power line 800 (FIG. 1). Conversely, for a given insulatingmaterial 825 (FIG. 1) having a maximum voltage breakdown rating, theapplied voltage on power line 800 may be increased, relative to thatpermissible when sharp corners are present.

Secondary winding 820 is shown in FIG. 5 with a single pass through thecore. In practice, secondary winding 820 may be configured as a coil,wound around a portion of the core.

There is thus provided an inductive coupler for coupling a signal to apower line. The inductive coupler includes (a) a magnetic core forplacement about the power line, (b) a coil wound around a portion of themagnetic core, where the signal is coupled to the coil, and (c) asemiconducting coating that encapsulates the core and contacts the powerline. The core has a longitudinal end, and so the inductive coupler alsoincludes a rounded semiconducting body that covers the longitudinal endand is in electrical contact with the semiconducting coating. The coilhas a lead emerging from the core, and so the inductive coupler alsoincludes a semiconducting layer over the end, to reduce electricalstress between the power line and a surface of an insulation coveringthe coil.

FIG. 6 is a vertical cross-section through an arrangement employing aninductive coupler having a semiconductive coating. An air path 1200 issusceptible to ionization and breakdown between power line 800 and asurface 1210 of an insulating layer 1225 surrounding a groundedsecondary winding 1220. A potential difference between power line 800and secondary winding 1220 is capacitively divided between air path 1200and insulating layer 1225. A greater proportion of the potentialdifference occurs across air path 1200 as compared to the potentialdifference across insulating layer 1225, and air path 1200 is also thepoorer insulator.

To mitigate this situation, a technique similar to that used in stresscones is employed. A stress cone is used at the termination of cableshaving two conductors and provides a gradual decrease of electricpotential so as to reduce field concentrations that might lead toinsulation breakdown. This is illustrated on the right half of FIG. 6.Embedded in insulating layer 1225, a semiconducting layer 1230 issandwiched between secondary winding 1220 and a surface 1215 ofinsulating layer 1225, and connected to coating 905 of core sections 805and 810. Semiconducting layer 1230 includes a combination of seriesresistance and stray capacitance 1235 that causes potential to decreasewith distance from the longitudinal end of the semiconductive corecoating 905, avoiding any excessive electrical stress concentration atthe distal edge 1240 of semiconducting layer 1230. Semiconducting layer1230 thus raises the potential of surface 1215 to be close to theprimary potential of power line 800, greatly reducing the potentialdifference across air path 1205, and preventing breakdown atunacceptably low primary voltages on power line 800.

Secondary winding 1220 is shown in FIG. 6 with a single pass through thecore. In practice, secondary winding 1220 may be configured as a coil,wound around a portion of the core.

Eliminating large potential differences across air paths and eliminatingpoints of high electrical stress can be achieved by a combination oftechniques. In one technique, the cores are coated by a semiconductinglayer, as described above in association with FIGS. 4 and 5. For anothertechnique, a section of high voltage cable is employed or speciallymolded for the coupler. The cable has an external semiconducting layerthat is energized by conductive or capacitive contact with coatedmagnetic cores. The cable has a center conductor that is grounded. Atthe two ends of a secondary winding, stress cones provide a terminationof the cable. Indoor stress cones without sheds may be used if thesecondary is embedded in insulation. Otherwise, outdoor stress coneswith sheds to increase the leakage path may be used.

FIG. 7 is a cross section of another embodiment of a high voltageinductive data coupler 1345, in accordance with the present invention.Coupler 1345 uses a high voltage cable as a secondary winding.

Power line 800 passes through core section 805, which is coated by asemiconducting layer 900. A secondary winding 1300, i.e., an innerconductor of a secondary cable 1305, is grounded via chokes (not shown),and passes through a core section 810, which is encapsulated in asemiconducting layer 905. Secondary cable 1305 is coated with asemiconducting layer 1310, which connects to a semiconducting portion1315 of a stress cone 1320. The entire lower portion of coupler 1345 isencapsulated in an insulating body 1325, equipped with sheds 1330 toprovide a long leakage path between power line 800 and groundedsecondary winding 1300.

Functionally, power line 800, or its thin insulation, contactssemiconducting layer 900 and brings the potential of semiconductor layer900 close to the potential of power line 800. The terms “gap” and “airgap” are used to indicate a non-magnetic spacer or non-magnetic regionbetween parts of a core, to increase current handling capacity andmaximum magnetomotive force before saturation. Semiconducting layer 900contacts semiconducting layer 905 at a gap 1350 between core sections805 and 810, respectively, bringing semiconducting layer 905 close tothe potential of power line 800. Secondary cable 1305 has itssemiconducting layer 1310 in direct contact with semiconducting layer905, thus also bringing semiconducting layer 1310 to a potential closeto that of power line 800.

At each end of secondary cable 1305, a stress cone 1320 terminatessecondary cable 1305, allowing secondary winding 1300 to exit coupler1345 without undue local electrical stress. An air path 1340 does notbridge a high potential, as the potential of the surface of coupler 1345is near the potential of power line 800 due to the underlying energizedsemiconducting layer 1310.

Secondary cable 1305 is shown in FIG. 7 with a single pass through thecore. In practice, secondary cable 1305 may be configured as a coil,wound around a portion of the core.

There is thus provided another embodiment of an inductive coupler forcoupling a signal to a power line. The inductive coupler includes (a) amagnetic core for placement about the power line, (b) a coil woundaround a portion of the magnetic core, where the signal is coupled tothe coil, and (c) a semiconducting coating that encapsulates the coreand contacts the power line. Furthermore, the coil has a section of highvoltage cable coated with semiconducting material, the semiconductingmaterial being in conductive or capacitive contact with semiconductingcoating, and inductive coupler also includes a stress cone at an end ofthe coil.

In a variation of the inductive coupler shown in FIG. 7, secondary cable1305 and semiconducting layer 1310 could be replaced by a coaxial cablehaving an outer conductor. Regardless of whether coupler 1345 isconfigured with secondary cable 1305 and semiconducting layer 1310 orwith a coaxial cable having an outer conductor, the outer layer of thesecondary winding is non-insulative. Generally, coupler 1345 includes(a) a magnetic core configured of core sections 805 and 810, forplacement about power line 800, and (b) a coil wound around a portion ofthe magnetic core. The coil includes the coaxial cable having thenon-insulative layer coupled to stress cone 1320, and the non-insulativeouter layer is at an electrical potential about equal to that of powerline 800 when inductive coupler 1345 is installed on power line 800.

It should be understood that various alternatives, combinations andmodifications of the teachings described herein could be devised bythose skilled in the art. The present invention is intended to embraceall such alternatives, modifications and variances that fall within thescope of the appended claims.

1. An inductive coupler for coupling a signal to a power line,comprising: a magnetic core for placement about said power line; and acoil wound around a portion of said magnetic core, wherein said coilincludes a coaxial cable having a non-insulative outer layer coupled toa stress cone, and wherein said non-insulative outer layer is at anelectrical potential about equal to that of said power line when saidinductive coupler is installed on said power line.
 2. The inductivecoupler of claim 1, wherein said non-insulative outer layer comprises aconductive material.
 3. The inductive coupler of claim 1, wherein saidnon-insulative outer layer comprises a semiconducting material.
 4. Theinductive coupler of claim 1, wherein said coaxial cable includes aninner conductor that is routed through said stress cone.
 5. Theinductive coupler of claim 4, wherein said inner conductor is connectedto ground.
 6. The inductive coupler of claim 4, wherein said innerconductor is connected to ground through a choke.
 7. The inductivecoupler of claim 1, further comprising a semiconducting coating thatencapsulates said magnetic core.
 8. The inductive coupler of claim 7,wherein said semiconducting coating contacts said power line.
 9. Theinductive coupler of claim 7, wherein said semiconducting coatingcontacts said non-insulative outer layer.
 10. The inductive coupler ofclaim 1, wherein said stress cone includes a semiconducting portion incontact with said non-insulative outer layer.
 11. The inductive couplerof claim 1, wherein said signal is coupled between said coil and saidpower line via said magnetic core.
 12. The inductive coupler of claim 1,wherein said magnetic core comprises a first portion and a secondportion with an air gap therebetween.
 13. An inductive coupler,comprising: a magnetic core having an aperture through which a powerline is routable; a cable having a center conductor routed through saidaperture, and having a portion external to said aperture; a stress conethrough which said center conductor is routed; and a semiconductingcoating that (a) encapsulates said magnetic core and said portion ofsaid cable that is external to said aperture, and (b) contacts saidstress cone, wherein said semiconducting coating contacts said powerline when said inductive coupler is installed on said power line. 14.The inductive coupler of claim 13, wherein said inductive couplercouples a data signal between said coil and said power line via saidmagnetic core.
 15. The inductive coupler of claim 13, wherein saidsemiconducting coating is at an electrical potential about equal to thatof said power line when said inductive coupler is installed on saidpower line.
 16. The inductive coupler of claim 13, wherein said innerconductor is connected to ground.
 17. The inductive coupler of claim 13,wherein said inner conductor is connected to ground through a choke. 18.The inductive coupler of claim 13, wherein said magnetic core comprisesa first portion and a second portion with an air gap therebetween.